In a conventional high frequency RFID system comprising at least one RFID reader and multiple RFID transponders (e.g. operating at 13.56 MHz, with data transmission from the RFID reader to the RFID transponder by means of phase modulation, and from the RFID transponders to the RFID reader by means of load modulation) electric energy is transmitted via an electromagnetic field from the reader to the transponders in order to supply the transponders with energy. In order to enable the transponders to receive the energy with a high energy level; their transponder air interfaces should be designed to have a high quality factor. However, disadvantageously, a very high quality factor has a negative influence on the whole RFID system insofar as it makes it difficult to achieve a very high data transmission rate between the transponders and the reader. The reason for this behavior of the RFID system is that increasing the quality factor of the air interface of a transponder is equivalent to reducing the frequency band width in respect of a given center frequency, which results in longer swing-out transients of various oscillating circuits employed in the RFID system. As a consequence, in known RFID systems a compromise between an extent of the quality factor and an intended data rate between the transponder and the reader has to be made.
Further, national and international standards limit both the theoretically available frequency bandwidths and the energy levels of signals being transmitted in the RFID systems, thereby barring a possible solution of the reciprocal relation between quality factors and data rates in RFID systems.
However, future applications of RFID systems (in particular Near Field Communication Systems (NFC)) will depend on higher data transmission rates than hitherto have been achievable. An example of such an application could be an electronic passport system having photographs, fingerprints and other biometric data stored in a built-in RFID transponder, which also requires fast data communication from the RFID reader to the RFID transponders for interrogating the RFID transponders quickly.
FIG. 1 shows a block circuit diagram of a prior art demodulator 10 for demodulating phase modulated input signals PM. The demodulator shown in FIG. 1 comprises a multiplier 11 for multiplying the phase modulated input signal PM with a cosine reference signal RCOS. The output signal P1 of the multiplier 11 is fed to a matched filter 12 and the filtered output signal P2 of the matched filter 12 is fed to a threshold detector 13 which compares the amplitude of the filtered signal P2 with a threshold and—depending whether the amplitude of the filtered signal P2 is above or below said threshold—determines the current binary state B of the filtered signal P2. A disadvantage of this known implementation is that the threshold detector 13 is very sensitive to distortions of the phase modulated input signal PM caused by high quality factors.
EP 0 045 260 B1 discloses a demodulator of signals modulated in accordance with a continuous phase modulation for the transmission of binary data. This document explains the principal means and processes employed in phase modulating/demodulating systems.